Graded reactive element containing aluminide coatings for improved high temperature performance and method for producing

ABSTRACT

A diffusion aluminide coating having a graded structure is applied over a nickel base superalloy substrate. The coating has an inner region of a diffusion aluminide adjacent to the substrate rich in a reactive element, typically Hf, Si or combinations of the two. The near surface region is a diffusion aluminide which is substantially free of reactive elements. Such coatings when used as bond coats in thermal barrier coating systems exhibit improved spallation performance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to gas turbine engines, and moreparticularly, to coatings applied to airfoils in the turbine portion ofthe gas turbine engine.

[0003] 2. Discussion of Prior Art

[0004] The current coatings used on airfoils exposed to the hot gases ofcombustion in gas turbine engines for both environmental protection andas bond coats in thermal barrier coating (TBC) systems include bothdiffusion aluminides and MCrAlY(X) coatings. These coatings are appliedover substrate materials, typically nickel-base superalloys, to provideprotection against oxidation and corrosion attack. These coatings areformed on the substrate in a number of different ways. For example, anickel aluminide (NiAl) layer may be grown as an outer coat on a nickelbase superalloy by simply exposing the substrate to an aluminum richenvironment at elevated temperatures. The aluminum diffuses into thesubstrate and combines with the nickel to form an outer surface of NiAl.A platinum modified nickel aluminide coating can be formed by firstelectroplating platinum to a predetermined thickness over thenickel-based substrate. Exposure of the platinum-plated substrate to analuminum-rich environment at elevated temperatures causes the growth ofan outer region of NiAl containing platinum in solid solution. In thepresence of excess aluminum PtAl₂ phases may precipitate in the NiAlmatrix as the aluminum diffuses into and reacts with the nickel andplatinum. Of course, an overlay of MCrAlY where M is an element selectedfrom the group consisting of Ni and Co and combinations thereof may beapplied to the substrate as a bond coat or as an environmental coatingby any known technique. When applied as bond coats in thermal barriersystems, an additional thermally resistant coating such asyttria-stabilized zirconia (YSZ) is applied over top of the coating.However, as the increased demands for engine performance elevate theengine operating temperatures and/or the engine life requirements,improvements in the performance of coatings when used as environmentalcoatings or as bond coatings are needed over and above the capabilitiesof these existing coatings. Because of these demands, a coating that canbe used for environmental protection or as a bond coat capable ofwithstanding higher operating temperatures or operating for a longerperiod of time before requiring removal for repair, or both, isrequired.

BRIEF SUMMARY OF THE INVENTION

[0005] The present invention is directed to an improved coating for useon an airfoil in the turbine portion of a gas turbine engine. In itsbroadest embodiment, the coating is for application over a nickel basesuperalloy substrate requiring environmental protection and which issubjected to elevated temperatures. The coating is comprised of adiffusion aluminide coating selected from the group consisting of NiAland PtAl. As used herein, unless otherwise specified, the term platinumaluminide or PtAl refers to a platinum-modified NiAl in which the NiAlincludes platinum in solid solution and in which PtAl_(x) phases may bepresent as precipitates in the matrix. This platinum aluminide may besubsequently modified as discussed herein. This coating is applied bytwo distinct diffusion aluminiding cycles resulting in two distinctregions. One region of the coating is a diffusion aluminide having atleast one element selected from the group consisting of hafnium (Hf),zirconium (Zr), yttrium (Y) and silicon (Si), and combinations of theseelements. The second region of the diffusion aluminide coating adjacentto the first region is substantially free of any of the elementsselected from the group consisting of Hf, Zr, Y and Si, except as occursas a result of natural diffusion processes during subsequent hightemperature exposures. Airfoils having coatings with such oxygen-activeelements as Hf, Si, Y, and Zr and combinations thereof exhibit longerlife and are capable of withstanding higher temperatures. When used forenvironmental protection, these coatings are expected to exhibit lesscorrosion and oxidation at higher temperatures than either the prior artaluminides or MCrAlY coatings. When used as a bond coat underlying athermal barrier coating, the bond coat of the present invention providesan advantage of improved damage resistance in terms of lower spallationvalues for comparable times as compared to a standard aluminidebaseline. This improvement translates into longer life.

[0006] Another advantage of the present invention is that it may beapplied to improve the performance of bond coatings or environmentalcoatings for either new airfoils or for airfoils requiring refurbishmentafter removal from service. Other features and advantages of the presentinvention will be apparent from the following more detailed descriptionof the preferred embodiment, taken in conjunction with the accompanyingdrawings which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is the tip end of an airfoil of the present inventionhaving the coating of the present invention applied as an environmentalcoating;

[0008]FIG. 2 is the tip end of an airfoil of the present inventionapplied as a bond coat with thermal barrier coating applied as a topcoat.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009]FIG. 1 shows the tip 28 of an airfoil having an environmentalcoating 30 applied over a substrate 32. The substrate 32 may be any highstrength nickel-based material capable of operation at elevatedtemperatures over long periods of time. However, it is preferred thatsubstrate 32 be a nickel based superalloy substrate having a nominalcomposition in weight percent of about 7.0% chromium, 1.5% molybdenum,5.5% tungsten, 6.2% aluminum, 7.5% cobalt, 0-0.15% hafnium, 3% rhenium,6.5% tantalium, 20-300 ppm yttrium, 40 ppm boron, 0.05% carbon and thebalance nickel and incidental impurities. Overlying substrate 32 in theembodiment shown in FIG. 1 is an aluminide coating 30 applied for thepurposes of environmental protection. The aluminide coating has twodistinct regions: a near surface region 36 and a region 34 immediatelyadjacent to substrate 32. Regions 34 and 36 comprise the aluminidecoating 30. Coating region 34 immediately overlying the substrate 32 isa diffusion aluminide coating selected from the group consisting ofplatinum aluminide and nickel aluminide, and contains hafnium (Hf),zirconium (Zr), yttrium (Y), and silicon (Si) either individually or incombination. Near surface region 36 is substantially the same as theregion of the coating 34 immediately adjacent to the substrate 32,except that the near surface region 36 is substantially free of Hf, Zr,Y, Si, and/or combinations thereof. The near surface region of thecoating can be very thin, having a thickness of as little as 5 μm in theexamples that follow. However, the thickness of the near surface regioncan be increased by lengthening the time the article is exposed to acoating method free of Hf, Zr, Y, Si, and/or combinations thereof. Asused herein the term “substantially free” is used to compare the nearsurface region 36 to the portion of the coating 34 immediately adjacentto the substrate 32. As the portion of the coating adjacent to thesubstrate may contain Hf, Zr, Y, Si, and/or combinations thereof inconcentrations greater than 2%, the near surface is considered to besubstantially free of these elements when their concentration is wellbelow the concentration in the coating adjacent to the substrate. Whilesmall amounts of these elements may be present in the near surfaceregion 36 it is the result of natural diffusion processes that occurduring subsequent high temperature exposures. However, the concentrationof these elements in the near surface region 36 due to the effects ofdiffusion is small as compared to the amount in the region adjacent tothe substrate. While this invention is described in terms of Hf, Zr, Yand Si, it will be understood that other strength increasing elementssuch as Ti, Re and Ta can also be added individually or in combinationwith any of the above elements. If any of these elements are added tothe coating 30, they are added in such a way that their concentration inthe region 34 immediately adjacent substrate 32 is significantly greaterthan their concentration in the coating in the near surface region 36.

[0010]FIG. 2 depicts the tip of a turbine blade 28 having a coating 30with the structure of the present invention, the region nearer thesubstrate 34 being an aluminide coating including reactive elements,either singularly or in combination, such as described previously, andthe near surface region 36 opposite the substrate being substantiallyfree of reactive elements. The overall coating thickness may vary fromabout 10 to 100 μm, with each of regions 36 and 34 varying from 5-30 μmeach. However, it is preferred that each of regions 36 and 34 vary from15-25 μm each. The concentration of the reactive elements is greater inthe region 34 of the coating adjacent the substrate than in near surfaceregion 36 opposite the substrate. A thermal barrier coating 40 may beapplied over top of the coating of the present invention as shown inFIG. 2. There are several methods for accomplishing the coatingstructure 30 of the present invention, shown in either FIG. 1 or FIG. 2.Each of these methods produces substantially the same product.

EXAMPLE 1

[0011] A substrate is electroplated with a thin region of platinum.After the article has been plated, it is placed in an atmosphere havinga source of aluminum and reactive elements Hf, Y, Zr, and Si, eitherindividually or in combination. Additional strengthening elements suchas Ti, Ta, or Re may also be present in the atmosphere. The atmospherecontaining a reactive element or elements and aluminum could be a vaporspecies of compounds of these elements such as halides as used in achemical vapor deposition process (CVD). Alternatively, theplatinum-plated article may be embedded in a mixture containing each ofthese elements as elemental powders or as various compounds plus anactivator, such as is done in a pack cementation process. Alternatively,the articles can be exposed to vapors containing these elements such asoccurs in above-the-pack (ATP) processes or in vapor phase aluminiding(VPA) processes. In this situation, the article is placed above theelements or compounds of the elements with an activator compound whilethe temperature is raised. As the various compounds begin to decomposeinto their constituent elements during heating, or as the elementalpowders are heated, the atmosphere experiences increasing concentrationsof the elements. Of course, the driving force for forming a nickelaluminide containing Pt in solid solution, is very strong. A diffusionregion of nickel aluminide with solid solution Pt begins to form firstand PtAl_(x) phases may precipitate in the NiAl. The other elements, Hf,Si, Y and Zr, as their concentration increases, are then incorporatedinto the outer nickel platinum aluminide. The nickel aluminide formationreaction and reactive element incorporation process are bothdiffusion-controlled processes. Because the driving force for reactingwith the aluminum-containing vapors is greater, the aluminide reactionproceeds more rapidly at first but after a period of time at elevatedtemperature, the concentration of reactive elements increases and theseelements are incorporated into the diffusion aluminide layer. Inaddition, the platinum aluminide phase PtAl_(x), precipitates, ifformed, will also have the reactive elements incorporated into theirstructure. The exact composition of the platinum aluminide precipitatedepends upon the concentration of the platinum, the time that thesubstrate is held at temperature, and the specific parameters of theheat treatment. Because of the lower driving force for the diffusion ofreactive elements, the reactive elements initially diffuse into region34 slower than the rate that the platinum aluminide coating is formed.In considering the diffusion rates of reactive elements, Hf diffusionoccurs at a relatively slow rate, while Si diffuses more rapidly. Y andZr diffusion occurs at rates intermediate between Hf and Si. Of course,the longer that the diffusion process is allowed to continue, the deeperthe penetration of the reactive elements will be into the diffusionaluminide region 34, with the faster diffusing elements diffusingfurther than the hafnium.

[0012] Because the reactive elements tend to oxidize in environmentssuch as the high temperature, corrosive environment of a turbine portionof a gas turbine engine, it is not desirable to expose region 34 to thisatmosphere. Therefore, after the aluminide region 34 has been formed bythe diffusion process and the reactive elements have been incorporatedinto it, region 36 is formed over region 34 by placing the coatedsubstrate into an aluminum-rich atmosphere. This atmosphere can becreated by any of the processing methods set forth above for forming thediffusion aluminide coating, except that the atmosphere is free of anyconcentrations of reactive elements. Thus, at an elevated temperature,region 36 is formed by a simple diffusion process. The driving force isthe interdiffusion of Al, Ni and Pt to form an outer region which issubstantially a platinum aluminide free of the slower diffusing reactiveelements. Thus, the substrate is coated with a diffusion aluminidecoating having an inner region including a platinum aluminide andreactive elements such as Hf, Zr, Y or Si, either alone or incombination, and an outer region of a platinum aluminide substantiallyfree of reactive elements. If the substrate is a nickel-base superalloy,the diffusion aluminide will also contain a diffusion zone or regionhaving the usual distribution of TCP phases, γ′ and carbides as well asnickel aluminide. There will be a gradient of these phases from thesubstrate to the outer surface.

[0013] Typically, the inner region 34 includes at least one reactiveelement in a concentration of 0-10%. In a preferred embodiment, Hf ispresent in a concentration, in weight percent, of about 0.25-10%, whileSi is optionally present in the amount of 0-5%.

[0014] While the platinum is deposited by an electroplating method inthe above example, it is understood by those skilled in the art that theplatinum may be deposited by any known method which deposits a thinregion of Pt, including physical vapor deposition processes, such assputtering and chemical vapor deposition.

[0015] Samples having the coating of the present invention applied asset forth in Example 1, were prepared by codepositing Al and Hf in apack process for 4 hours followed by vapor phase aluminiding in an Alatmosphere free of Hf. Unless otherwise noted, co-deposition (CODEP),was performed at about 1975° F. Samples having a platinum aluminidecoating and samples codeposited with Hf and Al over the substrate, buthaving no Pt, followed by vapor phase aluminiding in an Al atmospherefree of Hf were also prepared. The substrate for each sample was a ¼″pin sample of Rene′ N5, a Ni-based superalloy. The samples were thentested to failure by cycling them in a burner rig. Each cycle comprisesheating to about 1700° F. in 15 seconds, holding for about 10 minutes,heating to about 2075° F. in 15 seconds, holding for about 5 minutes,followed by a 75-second forced air cool. Testing was done in a mach 0.5velocity flame using salt-contaminated fuel. Table 1 presents thefailure results, in which the number of cycles required to attack theunderlying substrate was recorded. Samples prepared in accordance withExample 1, sample numbers 1, 2, 3, and 4 exhibited longer life than thePtAl control samples (9, 10 and 11) and samples not including Pt in thealuminide region (samples 5, 6, 7 and 8). TABLE 1 Cycles Sample CoatingType Processing Sequence to Failure  1 PtAl + Hf Pt-plate, Codep (1975°F.), VPA 1360   2 PtAl + Hf Pt-plate, Codep (1975° F.), VPA 800  3PtAl + Hf Pt-plate, Codep (1925° F.), VPA 1120   4 PtAl + Hf Pt-plate,Codep (1925° F.), VPA 1120   5 NiAl + Hf Codep, (1975° F.), VPA 400  6NiAl + Hf Codep, (1975° F.), VPA 400  7 NiAl + Hf Codep, (1925° F.), VPA480  8 NiAl + Hf Codep,(1925° F.), VPA 160  9 PtAl baseline Pt-plate,VPA 300 10 PtAl baseline Pt-plate, VPA 480 11 PtAl baseline Pt-plate,VPA 560

[0016] It should be noted that the samples of the present inventionwithout Pt noble metal incorporated (samples 5 through 8) exhibitedequivalent performance to the PtAl baseline samples (samples 9 through11). The incorporation of Hf using this invented process sequencecounteracts the removal of Pt, an element usually found to be necessaryfor erosion resistance.

EXAMPLE 2

[0017] A nickel aluminide coating is formed on the surface of anickel-based superalloy substrate by exposing the substrate to anatmosphere having a source of aluminum and reactive elements Hf, Y, Zr,and Si, either individually or in combination. Additional strengtheningelements such as titanium or tantalum may also be present in theatmosphere. The atmosphere containing the reactive element or elementsand aluminum can be a vapor species of compounds of these elements suchas halides as used in a chemical vapor deposition. Alternatively, thesubstrate may be surrounded with a mixture of powders containing each ofthese elements in elemental form or as various compounds. Typically,this is accomplished by placing the substrate directly into the powders,such as is done in a pack cementation process. Alternatively, thesubstrate can be exposed to vapors containing these elements such asoccurs in above-the-pack processes or in vapor aluminide processes. Inthis situation, the substrate is placed above the elements or compoundsof these elements while the temperature is raised. As the variouscompounds begin to decompose into their constituent elements duringheating or as the elemental powders are heated, the atmosphereexperiences an increasing concentration of the elements. Of course, thedriving force for forming the nickel aluminide is very strong, thereforea diffusion region of nickel aluminide begins to form on the surface.The other elements, as their concentration increases, are alsoincorporated into the thin but growing region of nickel aluminide. Thenickel aluminide formation reaction and reactive element incorporationprocess are both diffusion-controlled processes. Because the drivingforce for reacting with the aluminum containing vapors exceeds that forthe diffusion of the reactive elements, the reactive elements initiallydiffuse into nickel aluminide region 34, slower than the nickelaluminide region is forming. In considering the diffusion rates ofreactive elements, Hf diffusion occurs at a relatively slow rate, whileSi diffuses more rapidly. Y and Zr diffusion occurs at ratesintermediate between Hf and Si. Of course, the longer that the diffusionprocess is allowed to continue, the deeper the penetration of thereactive elements will be into the diffusion aluminide region 34, withthe faster diffusing elements diffusing to a greater depth than hafnium.

[0018] As in example 1 for the platinum aluminide diffusion region, itis undesirable for region 34 to be exposed to an oxidizing atmospheresuch as occurs in a gas turbine engine because the reactive elementssuch as Hf, Zr, Y and Si tend to rapidly oxidize, and the oxides are nottightly adherent. When these elements are present in highconcentrations, this oxidation is readily apparent because large areasof the surface become non-adherent (i.e. surface oxide spallation).After the reactive elements have been incorporated into region 34,region 36 is formed over region 34 by placing the coated substrate intoan aluminum rich atmosphere that is free of any concentrations ofreactive elements. Thus, at an elevated temperature, region 36 is formedby a diffusion process. The Al and Ni interdiffuse to form an outerregion which is substantially a nickel aluminide free of the slowerdiffusing reactive elements. Thus the substrate is coated with adiffusion aluminide coating having an inner region including a nickelaluminide and reactive elements such as Hf, Zr, Y or Si either alone orin combination, and an outer region of a nickel aluminide substantiallyfree of reactive elements. The diffusion aluminide region will containthe usual diffusion zone of TCP phases, and γ′ and carbides and therewill be a gradient of these phases from the substrate to the outersurface.

[0019] In each of these examples, region 34 of coating adjacent tosubstrate 32 is relatively rich in Hf, Si or any other reactive orstrengthening elements that are added. The coated substrate is nowplaced in an atmosphere of aluminum. Because aluminum is a much fasterdiffusing element than any of the reactive elements that that are addedin the previous step, the aluminum diffuses into the matrix and Pt or Nidiffuse outwardly toward the surface faster than the reactive elements.Thus, the outermost region of the coating is substantially richer inaluminum than in reactive elements such as Hf. The gradation of thecoating will be such that near surface region 36 will have a highconcentration of aluminum with little reactive elements such as Hf. Withincreasing distance inward from the outer surface towards the substrate,the reactive element concentration will increase to a peak level andthen begin to decrease until the reactive element (Hf, Y, Zr, and/or Si)concentration approaches zero near substrate surface 32 or to a valuethat is substantially less than their peak level concentrations. Whilethe inner region typically is formed by a pack cementation process inwhich there is a high concentration of a reactive element included, anyprocess that permits formation of a diffusion aluminide rich in areactive element may be used. Typically, ATP processes, chemical vapordeposition (CVD) processes or other vapor deposition methods may beused. Temperatures for forming the diffusion aluminide in theappropriate atmosphere range from about 1600° F. to about 2000° F. Thetime the article is held at the temperature will also vary, from about 2to about 20 hours. The time, temperature and atmosphere are allinterdependent, and will vary depending upon the desired results. Allthat is applicable to the formation of region 34 is equally applicableto the formation of region 36, although the atmosphere for region 36will be devoid of reactive elements.

[0020] The coating of the present invention has been discussed in termsof its applications as to a new turbine engine component, such as a newblade. However, this invention is not so limited. For example, thecoating can be applied to an existing component removed from servicefrom a gas turbine engine. If the component has a thermal barriercoating, the thermal barrier coating may be removed by any suitabletechnique, and the bond coat can be inspected. If present as anenvironmental coating region, the coating can be inspected. Ifnecessary, corrosion and oxidation products can be removed from the bondcoat or the environmental coating as set forth above. While it isnecessary to remove the corrosion and oxidation products from thecomponent requiring refurbishment, it is neither necessary nor desirableto remove any existing coating because the coating of the presentinvention can be incorporated directly into or over the existingcoating, whether the coating is an aluminide or an MCrAlY, where M is anelement selected from the group consisting of Ni and Co and combinationsof Ni and Co. The oxidation and corrosion products may be removed bytechniques well known in the art such as chemical cleaning, causticautoclave processing, or grit blasting. Of course, the aluminum contentof the remaining aluminide coating is depleted of aluminum, as some ofthe aluminum is incorporated into the oxide by-products and diffusedinto the substrate.

EXAMPLE 3

[0021] A Ni-based superalloy turbine blade having a nominal composition,in weight percent of 7.0% Cr, 7.5% Co, 0.05% C, 1.5% Mo, 3.0% Re, 0.15 %Hf, 6.2% Al, 5.0% W, 6.5% Ta, 160 ppm Y, 40 ppm B and the balance Ni andhaving an environmental coating of platinum aluminide is removed fromservice. Oxidation by-products and corrosion are removed from thesurface of the blade by grit-blasting. After cleaning, the airfoil isembedded in a powder mixture within a retort. The mixture contains asource of aluminum, a source of Hf, a halide activator and an inertfiller. The airfoil to be coated is heated in the range of about 1850°F. to about 2050° F., preferably about 1950° F.(1070° C.) in an inertatmosphere. The activator vaporizes and reacts with the aluminum source,such as an aluminum intermetallic or other aluminum-containing compoundto form an aluminum-rich halide vapor and a vapor of Hf. Aluminum reactswith the existing platinum aluminide coating to restore aluminum levels,while Hf also diffuses into the coating. The thickness and thecomposition of the coating depends on the time and temperature of theprocess, the activity of the powder and the composition of the workpiecebeing coated. After the coating has been regenerated by restoringaluminum, the airfoil is placed into a source of aluminum that is devoidof Hf, but including a halide activator and an inert filler. The airfoilto be coated is heated to a temperature in the range of about 1800-2050°F., preferably about 1950° F.±25° F. (1070° C.) in an inert atmosphere.The activator vaporizes and reacts with the aluminum source to form analuminum-rich halide vapor. Aluminum diffuses into the coating, butrelatively slowly, while Pt and Ni diffuses outward to form a platinumaluminide coating relatively free of Hf, since the Pt and Ni willdiffuse at a faster rate than the Hf. Thus the structure of the presentinvention is formed.

[0022] An airfoil component may then have the graded coating of thepresent invention applied to it as set forth in examples 1 or 2. If aplatinum aluminide coating is desired, then the methods set forth inexample 1 are followed. If the component is a nickel-base superalloy anda nickel aluminide coating is desired, then the methods set forth inexample 2 are followed. If the airfoil component is one which isundergoing repair, and it is not desirable to strip the aluminidecoating from the airfoil, the repair procedure of example 3 is followed.It does not matter whether the prior existing coating is a nickelaluminide or a platinum aluminide.

[0023] Articles having the graded coating of the present invention usedas a bond coat in a thermal barrier coating system are expected toexhibit superior spallation performance as compared to articles havingthe conventional PtAl bond coat and a ceramic topcoat of conventional 7%yttria-stabilized zirconia (7YSZ). In the following examples, buttonsamples of a nickel-base superalloy substrate having the same nominalcomposition as the substrate of example 3, with overlying bond coats and7YSZ topcoats were prepared and tested for spallation performance. Thespallation performance was compared to the spallation performance of thesame substrate having a standard PtAl coating as the baseline. Thespallation performance of this baseline substrate with this standardbond coat and a 7YSZ topcoat, measured when 20% of the thermal barriercoating spalls from the surface, was 230 cycles at 2125° F. for a onehour furnace cycle test ( FCT), or alternatively stated, an FCT life of230 cycles.

EXAMPLE 4

[0024] A button sample of the nickel-base superalloy substrate havingthe same nominal composition as the substrate of example 3 was preparedby embedding the substrate in a powder mixture of pure Hf, an NH₄Factivator, an aluminum powder source and alumina filler powders.

[0025] The Hf was present in the amount of 0.15-0.5% by weight, thehalide activator in the amount of 0.1-0.2% by weight, the aluminumpowder source in the amount of 1-5% by weight of codep-aluminide and thebalance alumina filler. The powders were thoroughly mixed. The retortwas heated to a temperature of about 1950° F. and held at temperaturefor about 4 hours to allow formation of a NiAl aluminide containing Hf.The sample was then removed from the pack and vapor phase aluminided, byflowing aluminum halide gas over the surface at a temperature of about1950° F. for about 4-8 hours. This vapor phase process deposits aluminumon the surface of the article. However, the driving force for thediffusion process is such that nickel diffused outwardly to form anouter region of NiAl relatively free of the slower-diffusing hafnium sothat the duplex coating of the present invention was formed. The buttonsample was then coated with a topcoat of 7YSZ and tested for spallationperformance. The sample exhibited an FCT life of 400 cycles which was asignificant improvement over the baseline sample. Microprobemeasurements on an as-coated section removed from samples prior totesting disclosed the presence of, in weight percent, 34% Al and only0.11% Hf in the first 5 μm of the bond coat. The Hf content increased to0.51 in the region between 5-15 μm, with 32% Al, and to 6.9% Hf in theregion between 15-30 μm, with an aluminum content of about 20.7%.

[0026] In this example, the aluminum codep powder consisting essentiallyof an aluminum intermetallic was used. However, any other suitablealuminum compound in powder form may be used.

EXAMPLE 5

[0027] Two button samples of the nickel-base superalloy substrate havingthe same nominal composition as the substrate of example 3 were preparedby first electroplating the sample with a thin region of Pt, about 3-6μm thick. The Pt-coated specimens were then subjected to a conventionaldiffusion heat treatment. The coated samples were then embedded in apowder mixture of pure Hf, an NH₄F activator, an aluminum powder sourceand alumina filler powders. The Hf was present in the amount of0.15-0.5% by weight, the activator in the amount of 0.1-0.2% by weight,the aluminum powder source in the amount of 1-5% by weight ofcodep-aluminide and the balance alumina filler. The powders werethoroughly mixed. The retort was heated to a temperature of about 1950°F. and held at temperature for about 5 hours to allow formation of aplatinum-modified nickel aluminide containing Hf. The samples were thenremoved from the pack and vapor phase aluminided by flowing aluminumhalide gas over the surfaces at a temperature of about 1950° F. forabout 4-8 hours. This vapor phase process deposited aluminum on thesurface of the article. However, the driving force for the diffusionprocess is such that Pt and Ni diffused outwardly to form an outerregion of platinum aluminide relatively free of the slower-diffusinghafnium so that graded coating of the present invention was formed. Thebutton samples were then coated with a topcoat of 7YSZ and tested forspallation performance. The two samples exhibited FCT lives of 320cycles and 380 cycles respectively, which represent significantimprovements over the baseline sample. Microprobe measurements of theas-coated samples failed with the lives disclosed indicated that in thefirst 5 μm of the bond coat of the respective samples, Hf at 0.73% and1.3%, while Al was at 28.5% and 29.2%, and Pt was at 27.5% and 24.4%,respectively. The Hf content increased to 2.5% and 11% in the regionbetween 5-15 μm, with 28.7% and 25.1% Al, and 23.6% and 31.7% Pt,respectively. In the final 15-30 μm of the sample, Hf was measured at1.4% and 2%, while Al was 29.0% and 27.7%, and Pt was 20.5% and 29.2%,respectively. All compositions are given in weight percent, unlessotherwise noted.

EXAMPLE 6

[0028] Ni-based superalloy substrate samples having a nominalcomposition of 7.0% Cr, 7.5% Co, 0.05% C, 1.5% Mo, 3.0% Re, 0.15% Hf,6.2% Al, 5.0% W, 6.5% Ta, 160 ppm Y, 40 ppm B and the balance Ni werefirst plated with a thin coating of Pt by a conventional electroplatingprocess and then vapor phase aluminided to produce a single phase PtAlcoating on the substrates. The samples were then inserted into a packbed having a composition in weight percent of about 0.25% Hf, about 1%Codep powder as an aluminum source, about 0.25% Si and about 0.2% NH₄Factivator in an otherwise typical pack cementation process. Afterheating for about four (4) hours at about 1950° F., three samples werecoated with a thermal barrier topcoat of 7YSZ and subjected to a FCTtest at about 2075° F. Samples were also analyzed to determine thechemistry following the 4-hour temperature exposure at about 1950° F.Although treatment was performed at 1950° F. for about 4 hours, anysuitable combination of time and temperature may be used to achieve theresults set forth below. Typically, temperatures in the range of about1850° F. to about 2000° F. for times of about 2-6 hours are used.

[0029] The overall thickness of the resulting coating was about 0.004″.The tested chemistry (in weight percent) measured from the near (orouter surface) was as follows: Region Below Surface Hf Al Si Pt 0-5 μm0.08 21.34 4.45 26.8 5-15 μm 0.31 20.99 4.80 24.9 15-30 μm 0.36 21.444.15 25.02

[0030] The samples subjected to FCT failed at 900 cycles, 1060 cyclesand 880 cycles. Spallation at failure for each sample was about 70% ofthe TBC, as compared to about 90% of the TBC for the baseline. Thus,samples prepared in accordance with this procedure had, on average abouttwice the cyclic spallation life as baseline samples.

[0031] Although the present invention has been described in connectionwith specific examples and embodiments, those skilled in the art willrecognize that the present invention is capable of other variations andmodifications within its scope. These examples and embodiments areintended as typical of, rather than in any way limiting on, the scope ofthe present invention as presented in the appended claims.

What is claimed is:
 1. A turbine airfoil, comprising: a nickel-basesuperalloy substrate; and a diffusion aluminide coating overlying thesubstrate, the aluminide coating selected from the group consisting ofPtAl and NiAl, and combinations thereof, the aluminide coating havingtwo regions, a first region of aluminide including at least one elementselected from the group consisting of Hf, Zr, Y, and Si and combinationsthereof, and a second region adjacent to the first region substantiallyfree of elements selected from the group consisting of Hf, Zr, Y and Si.2. The airfoil of claim 1 wherein the first region is an inner regionpositioned between the nickel-base superalloy substrate and the secondregion, while the second region is an outer region.
 3. The airfoil ofclaim 2 wherein the inner region has a thickness of from about 5 toabout 50 μm and the outer region has a thickness of from about 5 toabout 50 μm.
 4. The airfoil of claim 2 wherein the inner region has athickness of from about 15 to about 25 μm and the outer region has athickness of from about 15 to about 25 μm.
 5. The airfoil of claim 2wherein the concentration of elements in the inner region includes fromabout 0.25%-10% by weight Hf and from 0 to about 5% by weight Si.
 6. Acoating for application over a nickel-base superalloy substrate thecoating comprising: a diffusion aluminide coating selected from thegroup consisting of PtAl and NiAl and combinations thereof, thealuminide coating having two regions, a first region of aluminideincluding at least one element selected from the group consisting of Hf,Zr, Y, and Si and combinations thereof, and a second region adjacent tothe first region substantially free of elements selected from the groupconsisting of Hf, Zr, Y and Si.
 7. The coating of claim 6 wherein thefirst region is an inner region positioned between the substrate and thesecond region, while the second region is an outer region.
 8. Thecoating of claim 6 wherein at least one element selected from the groupconsisting of Hf, Zr, Y, and Si and combinations thereof is present inthe inner region in concentrations of from 0.25-10% by weight.
 9. Thecoating of claim 8 wherein the concentration of elements in the innerregion includes from about 0.25%-10% by weight Hf and from 0 to 5% byweight Si.
 10. A process for applying over a nickel-base superalloysubstrate a diffusion aluminide coating selected from the groupconsisting of PtAl and NiAl and combinations thereof having two regions,a first region of aluminide including at least one element selected fromthe group consisting of Hf, Zr, Y, and Si and combinations thereof, anda second region adjacent to the first region substantially free ofelements selected from the group consisting of Hf and Si, comprising thesteps of: placing the substrate into an elevated temperature atmospherehaving a high concentration of aluminum and at least one elementselected from the group consisting of Hf, Zr, Y and Si and combinationsthereof for a sufficient period of time for formation adjacent to thesubstrate of a first region of aluminide coating including at least oneelement selected from the group consisting of Hf, Zr, Y and Si; and thenremoving the coated substrate from the elevated temperature atmosphereand placing the coated substrate into an elevated temperature atmospherehaving a high concentration of aluminum and substantially free ofelements selected from the group consisting of Hf, Zr, Y and Si for asufficient time to allow for formation over the first region of a secondregion of aluminide coating substantially free of elements selected fromthe group consisting of Hf, Zr, Y and Si.
 11. The process of claim 10wherein the diffusion aluminide coating is applied by pack cementationby placing the nickel-base superalloy substrate into a mixture ofpowders including 0.15-0.50% by weight of an element selected from thegroup consisting of Hf, Zr, Y and Si, 1-5% by weight aluminum source,0.1 -0.2% by weight halide activator and the balance inert filler at atemperature for 1850-2050° F. for 2-6 hours to form the first region ofdiffusion aluminide coating adjacent to the substrate having at leastone element selected from the group consisting of Hf, Zr, Y and Si; nextremoving the substrate from the elevated temperature atmosphere; thenrealuminiding the diffusion aluminide-coated substrate by placing thesubstrate into the atmosphere having a high concentration of aluminumsubstantially free of elements selected from the group consisting of Hf,Zr, Y and Si at a temperature of from about 1850-2050° F. for asufficient time to allow for formation over the first region ofdiffusion aluminide coating of a second region of aluminide coatingsubstantially free of elements selected from the group consisting of Hf,Zr, Y and Si having a thickness of at least 5 μm.
 12. The process ofclaim 11 wherein a thin region of platinum is first electrodeposited onthe nickel base superalloy substrate before the step of placing thesubstrate in the mixture of powders.
 13. The process of claim 12 whereinthe platinum coated substrate is heat treated at temperature of1600-2000° F. for 2-20 hours to allow platinum to diffuse into thesubstrate prior to the step of placing the substrate in a mixture ofpowders.
 14. The process of claim 10 wherein the nickel-based superalloysubstrate is comprised of a turbine airfoil removed from service furtherincluding removing oxide products and corrosion products from thesurface of the airfoil substrate prior to the step of placing theairfoil into the elevated temperature atmosphere having a highconcentration of Al and at least one element selected from the groupconsisting of Hf, Zr, Y and Si and combinations thereof.
 15. A processfor applying over a nickel-base superalloy substrate a diffusionaluminide coating selected from the group consisting of PtAl and NiAland combinations thereof having two regions, a first region of aluminidesubstantially free of elements selected from the group consisting of Hfand Si and a second region of aluminide adjacent to the first regionincluding at least one element selected from the group consisting of Hf,Zr, Y, and Si and combinations thereof, comprising the steps of: placingthe substrate into an elevated temperature atmosphere having a highconcentration of aluminum and substantially free of Hf, Zr, Y and Si fora sufficient period of time for formation adjacent to the substrate of afirst region of an aluminide coating; then removing the coated substratefrom the elevated temperature atmosphere and placing the coatedsubstrate into an elevated temperature atmosphere having a highconcentration of aluminum and at least one element selected from thegroup consisting of Hf, Zr, Y and Si and combinations thereof for asufficient time to allow for formation over the first region of a secondregion of aluminide coating; and holding the coated substrate at anelevated temperature for a sufficient time so that the elements selectedfrom the group consisting of Hf, Zr, Y and Si, diffuse from the secondregion into the first region.
 16. The process of claim 15 furtherincluding the step of coating the nickel-base superalloy substrate witha thin region of Pt, and wherein the diffusion aluminide coating isformed by: first forming a single phase PtAl-coated substrate by vaporphase aluminiding the substrate to form a first region of aluminidecoating; then placing the coated PtAl-coated substrate into a packpowder bed having a composition of, in weight percent, about 0.25% Hf,about 1% aluminum-containing powder, about 0.2% halide activator andoptionally about 0.25% Si at a temperature in the range of about 1850°F.-2000° F. for about 2-6 hours to allow for formation over the firstregion of a second region of aluminide coating so that Hf, andoptionally Si, diffuse from the second region into the first region.